Purified terephthalic acid (English ab. PTA) product is the raw material of polyethylene glycol terephthalate (polyester for short, English ab. PET).
Prepare the purified terephthalic acid (PTA) with paraxylene (PX) as raw material, compressed air or other oxygen-containing gases (oxygen for instance) as auxiliary material, acetic acid as solvent, cobalt acetate and manganese acetate as catalysts, and hydrobromic acid as cocatalyst, carry out the oxidation reaction at the relatively gentle temperature to obtain terephthalic acid (TA). The ion contents of cobalt, manganese and bromine—effective ingredients—in catalyst and cocatalyst generally range from 900 ppm to 2000 ppm, the ratio between cobalt, manganese and bromine is a little higher, generally in 1:1:1 or 1:2:3 or 1:2:2 and so on.
After the processes of oxidation reaction, terephthalic acid (TA) crystallization, separation, desiccation, etc., obtain the crude terephthalic acid (CTA, or TA), then dissolve the CTA in hot water, and send it to hydrogenation reactor at both high temperature and high pressure, carry out the hydrogenation reaction in the presence of Pd/C catalyst, reduce the 4-CBA into P-methyl benzoic acid (PT acid), the water solubility of PT acid is significantly higher than TA, so the TA and PT acid can be basically separated by the re-crystallizing of TA in aqueous solution. Then the purified terephthalic acid (PTA) is obtained by the water washing, centrifugal separating, filtering and drying of the TA. The PT acid and a little of TA in aqueous solution are returned to the oxidation reactor after which are concentrated and dehydrated.
The known research indicated that the oxidation reaction from paraxylene (PX) to terephthalic acid (TA) was a tandem reaction, it mainly underwent four reaction processes, firstly it was from PX to P-methyl benzaldehyde (TALD), secondly it was from TALD to PT acid, thirdly it was from PT acid to 4-CBA, fourthly it was from 4-CBA to TA. Four of the oxidation reactions were Irreversible reactions. Wherein the reaction velocity from PT acid to 4-CBA was the slowest under the same conditions, this reaction was the control step of the whole tandem reaction (see Reference: Weizheng Sun, et al, Kinetics of Liquid-Phase Catalytic Oxidation of p-Toluic Acid. Chemical Reaction Engineering and Technology. February, 2007. Issue 1, Vol. 23, p 8-12).
The design philosophy of this invention is as follows:
1. As to different reaction steps, offer the different reaction conditions, decrease the oxidation reaction temperatures from PX to p-methyl benzaldehyde (TALD) and from p-methyl benzaldehyde (TALD) to PT acid whose oxidation reaction conditions are not too rigorous, reduce the side reaction during which the solvent acetic acid is oxidized into carbon dioxide or carbon monoxide at high temperature as much as possible;
2. Increase the oxidation reaction temperature of control step of tandem oxidation reaction (from PT acid to 4-CBA), so as to increase the reaction velocity in this control step, then achieve the aim of increasing the whole oxidation reaction velocity;
3. Make sure that TA cannot form any crystal inside oxidation reaction, choose water as a secondary solvent during the oxidation processes from PT acid to 4-CBA and to TA, increase the temperature to enhance the TA solubility, but never enhance the acetic acid consumption.
According to this design philosophy, this invention innovated the designs for the new processes of reaction materials, separate the final oxidation product TA from PX and other intermediate products as much as possible, so that the main reaction materials can establish the upstream to downstream flow and processes from and to reactor. Instead of the current PX oxidation reactor inside, various materials, intermediate products and final product are mixed together, it should be a mixed oxidation reaction. This invention is a step oxidation reaction (or step-by-step reaction), this not only allows TA crystals to have no the impurities of 4-CBA and PT acid, but also allows the retention time of produced TA inside the oxidation reactor to shorten, thus reduting the raw materials consumption resulting from TA over-oxidizing and acetic acid consumption.